Microinverter vs String Inverter for Offgrid?

[myersfamilyhome]

I searched for a deep dive into the pros and cons of each in an offgrid setting but found very little. Hopefully this will fill the gap. Here are some of the questions I have.
* Is the output of microinverters pure sine wave?
* Can microinverters produce 120/240 (split phase) without additional hardware?
* What are the pros/cons specifically of one over the other in an offgrid scenario?
* Is anyone using microinverters offgrid? and how?
* How can microinverters integrate with an existing LifePo4 build?

 

[sunshine_eggo]

I searched for a deep dive into the pros and cons of each in an offgrid setting but found very little. Hopefully this will fill the gap. Here are some of the questions I have.
* Is the output of microinverters pure sine wave?

Of course. If they're not, they're some sort of obscure junk.

* Can microinverters produce 120/240 (split phase) without additional hardware?

No. They are single phase 240 are are most residential GT string inverters.

Doesn't matter because they're attached to the mains, and there's a neutral there.

* What are the pros/cons specifically of one over the other in an offgrid scenario?

Why are we talking about GT in an offgrid scenario?

* Is anyone using microinverters offgrid? and how?

I'm not, but they would need to be AC coupled with an off-grid inverter OR be capable of being powered by batteries - don't think any are.

* How can microinverters integrate with an existing LifePo4 build?

AC coupled to an off-grid inverter.

Generally speaking if you're off-grid, you should go with an off-grid inverter. If you find a good deal on used GT, and you want to expand your system, GT might make sense.

 

[myersfamilyhome]

In an offgrid scenario many of us have at least a couple miss matched panels or come across deals on panels that would be ideal for daytime only use. I have seen several examples where a single panel with a microinverter was powering multiple 120v devices. If the microinverters efficiency is great and it's able to utilize 100% of the panel output as opposed to a string inverter which is at the mercy of the lowest output panel in your array then in a large mismatched array you could be losing the amount of power comparable to several panels even in the most optimal configuration.

Also I can see value in having a separate array that provides power just during the day without the need for batteries. Ex. hot water heater, well pump, washer/dryer. In this scenario these items are power hungry and significantly impact the life of your batteries. If microinverters are used to power them separate from your battery build, you can reduce the overall cost/size of the batteries needed or extend the life of your batteries. Ultimately when an offgrid system is built, we ideally want to reduce the frequency of replacement on parts in it. The batteries are the most expensive and shortest lived part for most people. Using microinverters in the right application seems optimal

 

[sunshine_eggo]

In an offgrid scenario many of us have at least a couple miss matched panels or come across deals on panels that would be ideal for daytime only use. I have seen several examples where a single panel with a microinverter was powering multiple 120v devices. If the microinverters efficiency is great and it's able to utilize 100% of the panel output as opposed to a string inverter which is at the mercy of the lowest output panel in your array then in a large mismatched array you could be losing the amount of power comparable to several panels even in the most optimal configuration.

Also I can see value in having a separate array that provides power just during the day without the need for batteries. Ex. hot water heater, well pump, washer/dryer. In this scenario these items are power hungry and significantly impact the life of your batteries. If microinverters are used to power them separate from your battery build, you can reduce the overall cost/size of the batteries needed or extend the life of your batteries. Ultimately when an offgrid system is built, we ideally want to reduce the frequency of replacement on parts in it. The batteries are the most expensive and shortest lived part for most people. Using microinverters in the right application seems optimal

The key here then is your off-grid "main" inverter has to be capable of AC coupling, i.e, to use the surplus AC to charge the batteries; however, there are limits...

3kW of GT requires your off-grid inverter to be at least 3kW. This doesn't mean you get to add them together and get 6kW of power delivery. It's basically just another 3kW array added to your existing system that can power loads and charge your batteries...

Very little advantage over adding a GT inverter for another 3kW of solar vs. just getting another MPPT.

 

[Bluedog225]

Interesting. To test my understanding, on a 5 identical panel setup with IQ8 micros, if 4 micros are putting out 2 amps/120v each, and one is damaged or shaded, and putting out 1 amp, what is the total available current at the combiner box? Trying to learn this stuff. Thanks.

 

[sunshine_eggo]

Interesting. To test my understanding, on a 5 identical panel setup with IQ8 micros, if 4 micros are putting out 2 amps/120v each, and one is damaged or shaded, and putting out 1 amp, what is the total available current at the combiner box? Trying to learn this stuff. Thanks.

2+2+2+2+1 = 9

I had it stuck in my head that micro inverters were in series like optimizers, and that's just wrong. I edited my post to remove the offending material.

 

[myersfamilyhome]

Interesting. To test my understanding, on a 5 identical panel setup with IQ8 micros, if 4 micros are putting out 2 amps/120v each, and one is damaged or shaded, and putting out 1 amp, what is the total available current at the combiner box? Trying to learn this stuff. Thanks.

2+2+2+2+1 = 9 amps total from microinverters

In a string setup the total power sent to the mppt would be 5 x whatever the lowest producing panel is which would be definitely less than micro

 

[offgriddave]

Wait, I am confused!

I thought microinverters "grid tie" needed a "reference" circuit, 120/240 split phase to work? Was I wrong? Will they independently throw out 240v split phase? How would they synchronize? I was under the impression they worked by seeing the waveform and then pushing it up. If not, how do they work?

 

[sunshine_eggo]

Wait, I am confused!

I thought microinverters "grid tie" needed a "reference" circuit, 120/240 split phase to work? Was I wrong? Will they independently throw out 240v split phase? How would they synchronize? I was under the impression they worked by seeing the waveform and then pushing it up. If not, how do they work?

You are not confused. They need a source to synchronize to. Hence my multiple references to "AC coupling" where an off-grid inverter is providing the "grid" to which the micros sync. They will not just sit there and provide AC from panels.

 

[myersfamilyhome]

Enphase IQ8 microinverter to operate when the grid is down - AC Solar Warehouse

The IQ8 will supply electricity when the sun is shining even during a loss of electricity supply from the grid or simply when there is no grid at all.

www.acsolarwarehouse.com

 

[Bluedog225]

2+2+2+2+1 = 9

I had it stuck in my head that micro inverters were in series like optimizers, and that's just wrong. I edited my post to remove the offending material.

Great. That’s what I was hoping for. Thanks!